Transmission gain control



M, w35 L. G. ABRAHAM TRANSMISSION GAIN CONTROL Filed Dec. 20, 1954 lliPatented Apr. 14, 1936 UNITED STATES TRANSMISSION GAIN CONTROL LeonardGladstone Abraham, Madison, N. J., as-

signor to American Telephone and Telegraph Company, a corporation of NewYork Application December 20, 1934, Serial No. 758,524

9 Claims.

This invention relates to devices for. controlling the over-alltransmission loss in a communication transmission line, and moreparticularly to lines in which a pilot current is transmitted over atransmission line and its intensity or its energy level on reception isused as a measure of the net loss in the line. This pilot signal isIthen used to bring about adjustments to keep vthe over-all net loss ata substantially constant desired value.`

In certain cases where devices of this nature have been used, the pilotsignal has been a sustained frequency outside but near the speechfrequency band or other signal band to be transmitted. In other casesthe pilot signal has been a tone within the message frequency band whichis sent only intermittently and at moments not interfering with themessage signal. In both cases the necessary equipment has been spoken ofas a tonlar, an abbreviation of tone operated net loss adjustingreceiver.

The first of these has the advantage of being on continuously but hasthe disadvantage that its frequency is outside the signaling band to` betransmitted. Since the variations of the line characteristics at thatfrequency are not the same as the changes in frequency characteristicswithin the signaling band, there is imperfect regulation. The advantageof the second type is that it makes adjustment for a typical frequencyWithin the signaling band but it has the disadvantage that the circuitVaries between adjustments and it may do so suificiently to removeconsiderable of the advantage of the tonlar unless adjustments are madequite frequently.

The purpose of this invention is to combine the advantages of the twotypes of tonlars, that is, to obtain, continuously, substantiallycorrect compensation for variations at some selected frequency withinthe signaling band. I accomplish this by sending continuously afrequency just outside the signal band which supervises the line forthat frequency but the apparatus for which is itself adjusted orcalibrated from time to time by an occasional signal of frequency in thetransmitted signal band in such manner vthat the correction is really interms of the latter. Thus the variations that occur frequently at bothof these pilot frequencies are eliminated and the slow change in thetransmission characteristic between these two frequencies is compensatedfor before it becomes appreciable.

The invention will be better understood by reference to the followingspecification and the accompanying drawing, in which the ligure givesthe essential circuit arrangement for a tonlar possessing thecharacteristics which I desire.

To make the description more definite and to give an example of thefrequencies involved, it will be assumed that a speech signal is to betransmitted using the band of frequencies up to 3400 cycles. 'Ihen forthe continuous control frequency we may take 3500 cycles, and for theintermittent frequency we may take 1000 cycles. It is to be understood,however, that these frequencies are given for illustrative purposes onlyand are not intended in any way to limit the scope of the invention.

Referring to the figure, there is shown an incoming transmission line Lwith repeating coils and amplifiers, there being two shown here. Thegain of these amplifiers may be controlled at least in part, by asuitably located potentiometer P1, here shown as on the input of the rstamplifier, and the vpotentiometer is controlled by a clutch mechanism #Loperated by motor I2, to` raise or lower the contact of thepotentiometer P14, thus controlling the net loss of the circuit in amanner hereinafter described. The 3500 cycle current and occasional 1000cycle current which come over the transmission line are taken off at theoutput of amplifier #2 and impressed on the input of amplifier #3through an adjustable potentiometer P2. When 3500-cycle current alone ison the line it passes through an amplifier-detector I5, tuned to 3500cycles, and then controls therelay R1. If the current is too large, dueto a decrease in transmission loss, the relay R1 operates, causing thelower gain clutch of mechanism #I to be actuated, thus changing thesetting of P1 and restoring the 3500-cycle current at the terminal ofamplifier #2 to ya value such that the tongue of relay R1 is restored toits normal intermediate position. If, on the other hand, the 3500-cyclecurrent is too weak, the relay R1 closes on the back contact and theopposite effect is obtained, that is, the raise gain clutch of mechanism#I is actuated, thus again changing the setting of P1 to restore the3500-cycle current at the output of amplifier #2 to its normal value.The sensitivity of the relay R1 is controlled by a potentiometer P3across the output of amplifier #3, and by changing the setting of thispotentiometer, one controls the amount of 3500-cycles at the output ofamplifier #2 which is required to operate the clutch mechanism #I in theone direction or the other. When the occasional 1000-cycle current isadded, the circuit functions on the 3500-cycle current in the samemanner except that the 1000-cycle current may bring about a change inthe setting of the potentiometer P3 for the purpose of adjusting orcalibrating" the 3500-cycle circuit to cause the relay R1 to operateabout that average value of the 3500-cycle current which is appropriatein View ofthe existing transmission loss at 1000 cycles.

More specifically, it will be seen that the 1000- cycle current ispicked olf by an amplifier-detector I8, tuned to 1000 cycles. It thusoperates or releases the relay Rz, depending upon its strength, and socauses the clutch mechanism #2 to adjust the potentiometer P3, which inturn results in a corresponding adjustment in P1' until the desiredamount of G-cycle current is obtained at the output of amplifier #2.

After a suiiicient time interval to insure that this adjustment has beenmade (and this will be in the order of half a second), apulse-clippingcircuit operates to remove the ground from the tongue of relay R2 andprevents any further change in P3. This pulse-clipping circuit operatesas follows: n

A relay R3 is connected in the outpit 'of the amplifier-detector I8 inparallel to relay R2. Controlled by the contact of relay R3 are twoother relays, R4 and R5, Athese being in parallel with each other andeffective on the making of R3. The -relays R4 and R5 are both sensitivebut R5 is a slow-'operate relay, rendered so by resistances 2U andgrounded condenser 2|. When current due to the 100G-cycle is present inthe output of detector I8, R3 operates 'and in turn brings about theoperation of 'R4 and R5. The operation of R4 applies ground Vto thetonguev of R2. A short time later R5 operates to 'short-circuit R4,which in turn releases and the ground is removed from the tongue of R2.When the 100`Ucycle current is removed (and it is on for a short timeonly), relays R3 and then R5 release, the tongue of R2 is leftungrounded and` the cycle is ready to be repeated. It is desirable, ofcourse, that the relay R3 lshall not be responsive to any 100G-cyclevoscillations coming from the speech message itself. This isaccomplished, in practice, by arranging that the amplitude of theintermittent 10004cycle control current shall be of a magnitudecorresponding to testing conditions, that is, of a magnitude whichshould, in general, be large compared to any 1000 cycle not arising inconnection with the speech message. In order furthenhowever, to precludeoperation of this relay Rs by speech, it is desirable to make the relayR3 a slow-operate and quick-release relay, in which case, it will not be4operated except by the definite intermittent M500-'cycle controlcurrent.

rIt is apparent that many variations may be made in the combination ofrelays to bring about the desired.` results, all withoutL departing fromthe spirit of this invention. This is particularly true of thepulse-clipping circuit,` andthe particular arrangement shown is forillustrative purposesonly.

What is claimed is: Y. Y

l. In a system for maintaining substantially constant transmission lossover ay transmission line, comprising a source of sustained current ofone frequency and arsource of intermittent current of another frequency,each transmitted over the line as pilot currents, means responsive tothe sustained current to kadjust the gain in accordance with the Vnetloss at that frequency, and means responsive tothe intermittent currentto adjust the sensitivity of the first-named means in accordance withthe net loss at frequency of the intermittent current.

2. The methodf of adjusting tandem repeaters for gain in a linecomprising those repeaters', which consists in sending a continuouscurrent of one frequency and an intermittent current of anotherfrequency over the line, using the first current to adjust the gain ofthe repeaters in accordance with line variations at that frequency, andusing the second current to control the effectiveness of the first inaccordance with line variations at that second frequency.

3. The method of adjusting tandem repeaters for gain in a telephone linecomprising those repeaters, which consists in sending over the line acontinuous current of a frequency outside the signal frequency band andan intermittent current of a different frequency within the signal band,using the rst current to adjust the gain of the repeaters in accordancewith line variations at that frequency, and using the second currentV tocontrol the effectiveness of the first in accordance with linevariations in the speech band.'

4; Themethod of adjusting the transmission level in a line whichconsists in sending a con; tinuous tone frequency current andanintermittent tone frequency current over the line, selec'- tivelyseparating these from the line where the adjustment is to'be made,detecting these'currents and applying the first current to introducegain 'or 'loss in accordance with the intensity of the current of thatfrequency as detected, and applyingthe lsecond current to calibrate andcontrol the adjustments of 'the rst in accordance withline'variations atthe second frequency.

5. `In combination, a transmission line, repeat'- ers therein, means forsending 'oversaid line a sustained tone frequency current, means forsending over said' line a second intermittent tone frequency current',means for separating out and detecting the'S'efrequenCiesfOrtransmission into derived circuits, and means in' said 'derived 'cir'-cuits for adjusting the' gain -or loss of the line in accordance withline variations at the first frequency', and further means forcontrolling the adjustment by the' rst adjusting means in ac cordanc'ewith line'variati'ons at the second fre;-

quen'cy.'

6. A telephone line having 'a repeater station thereon, meansVcontrolled 'by aV tone having a frequency outside the'voice frequencyrange and transmitted Vcontinuously over the line for con'- trolling thegainto hold thev energy level beyond the repeater and at that frequencyvwithin 'desir'ed limits, and additional means controlledV by a tonehaving a frequency within the voice fre-'- quency range transmitted' atintervals over the line'for controlling the effectiveness of thefirstnanied control nieans in accordance with line variationsat'thfrequency of the intermittent tone.

'1. The Combination of claim ceharacterized by means for 'preventingopeationof the second= named control means by currents transmitteddiirng the intervals betweenthe transmission `of the intermittentfrequency tone.'

8.` The combination of 'claim`6 characterized by means comprising aslow-'actingirelay'for preventing operation 'ofthe secondnamed'controlmeans by Voice currents during communication over 'the line.

9. The combination of claim 6characterl'zed by the fact that there is asloW-acting'relay under control ofv detected intermittent frequencywhich'relay prevents operation of the secondnamed 'control 'means byvoice 'currents 'during communication over'tlie line.

LEONARD `ca '.ADs'roNn ABRAHAM.

